Drilling and completion technologies for deep carbonate rocks in the Sichuan Basin: Practices and prospects

Abstract

The exploration and development of oil and gas resources in the Sichuan Basin is shifting to the deeper strata. The deep and ultra-deep wells in deep carbonate gas reservoirs are faced with many difficulties, such as multiple pressure systems, poor drillability, ultra-high pressure, ultra-high temperature and high sulfur content, which bring great challenges to drilling and completion engineering. In order to ensure the smooth exploration and development of deep carbonate oil and gas resources in the Sichuan Basin, the overall concept of combining field practice and technological research is followed. During the 13th Five-Year Plan, a batch of ultra-deep wells (well depth about 8000 m) have been drilled fast and safely (such as Well SYX133 in the Shuangyushi structure), and great progresses have been achieved in the experimental research of the support technologies in drilling and completion of deep and ultra-deep wells. And the achievements are as follows. First, the optimization of non-standard well structure, combined with the application of precisely managed pressure drilling technology and under-pressure plugging technology, lays a foundation for the optimized fast drilling and the safe reaching of geological targets. Second, a high-efficiency customized PDC drill bit is comprehensively optimized, popularized and applied, which effectively improves the average rate of penetration (ROP) in difficult-to-drill formations. Third, the development and application of drilling fluids (e.g. being resistant to high temperature of 200 °C and anti-composite brine) and active under-pressure plugging technology effectively reduces downhole complexities while drilling the complex formations, such as high temperature and high pressure brine and circulation loss. Fourth, when precisely managed pressure drilling and precisely managed pressure cementing methods are applied in the strata with a narrow density window, multiple pressure systems and pressure sensitivity, the average drilling and completion fluid loss is reduced by more than 90%, and the complexity treatment time is cut down by more than 85%, and the cementing quality pass rate is increased by more than 20%. Fifth, the integrated application of ROP improvement technologies and tools (e.g. air drilling) results in a great reduction of drilling cycle and cost. It is concluded that the experimental research achievements provide basic support for the fast development of natural gas resources in the Anyue Gas Field, Sichuan Basin, and the great discovery of the Permian and Devonian natural gas resources in the northwestern Sichuan Basin. What's more, in order to adapt to the development of the deep natural gas (over 9000 m) in the Sichuan Basin during the 14th Five-Year Plan, it is necessary to speed up researches on drilling and completion technologies in nine aspects, e.g. high-temperature downhole tools and working fluid.